The main imaging components of a document scanner are often bundled together and manufactured as a “camera subassembly” by OEM manufacturers. The subassemblies can then be shipped to a final manufacturing site where they are integrated, or built into, the rest of the document scanner.
Dust contamination is always a concern when manufacturing camera subassemblies for installation into an imaging device, as it can cause a decrease in the quality of images captured with the device. Other contaminants, such as scratches on the glass cover, may also occur during manufacturing and similarly lead to reduced image quality. A typical subassembly is comprised of a CCD camera, reflectors, associated electronics, and a glass cover—all enclosed in an air tight plastic body. The subassembly may also comprise LED illumination. The imaging sensor could be a CIS sensor (as opposed to CCD), and the illumination could be CCFL (as opposed to LED). In particular, if dust particles get inside the subassembly during the manufacturing process they can move around and frequently end up on the underside of the glass surface. This can often be the result of shipping and handling of the camera subassembly or the fully assembled imaging device, such as a document scanner. Any dust particles present on the glass surface during scanning or image capturing, if in the right location and sufficiently large, can result in poor quality images. For example, the dust particles may show up as vertical streaks present in captured images.
A common technique for addressing streaks in the imaging path of a CCD camera is to perform an image calibration that accounts for the presence of the dust. However, the dust particles are not stationary and can move within the camera subassembly. If one or more dust particles move, the imaging device, such as a scanner, would then be calibrated to account for a streak that no longer exists while leaving streaks caused by the dust particles at their new positions unaccounted for. Because the subassembly is sealed, it may be difficult to remove the dust or fully compensate for its presence within the camera subassembly once the subassembly is installed into the imaging device.
What is needed is an accurate, automated method of detecting dust particles in the camera subassembly before it is accepted from the manufacturer and assembled in the imaging device. Currently, a visual inspection of the camera assemblies before installation is performed using direct illumination from an external source. However, this visual inspection technique has several drawbacks. It is very labor-intensive, dust particles can often be missed, and it is difficult to consistently apply criteria used in making a determination as to whether a subassembly is acceptable or not. For example, the extent of analysis and final determination may vary depending on the individual conducting the visual inspection, as each different inspector may apply different criteria, or some may be more thorough than others. Thus, an accurate, automated method of inspecting the camera subassembly for dust particles prior to installation into an image device is needed.